AbstractExperimental measurements have been conducted of Knudsen diffusion through a model porous medium fabricated by the pelleting of monodisperse silica spheres. Tortuosity factors for this random‐assemblage‐of‐spheres porous solid are calculated and compared with values obtained from both theory and measurements made in the bulk diffusion regime. Evidence is presented to support the use of a mean pore radius calculated as twice the ratio of pore volume to surface area, as opposed to values obtained from mercury porosimetry or sorption isotherm ana